Immunoglobulin variable genes are a large, multigene family whose members rearrange at different times in development. Early in mouse oncogeny, rearrangement of heavy chain variable genes (VH) is limited to a few genes that are located proximal to the joining genes. Although less is known about rearrangement of kappa light chain variable genes (VK), our preliminary data indicates that VK gene rearrangements are more heterogeneous than VH gene rearrangements and are not restricted to genes proximal to the joining genes. This contrast between the rearrangement patterns suggests that different developmental mechanisms may be used. It is therefore important to obtain more information on VK gene rearrangements. The research proposed in this application has three aims. (1) To identify VK genes that rearrange early in ontogeny. Rearranged genes from three sources of immature B cells will be examined by sequencing and hybridization to probes of VK subfamilies. (2) To determine if VK and VH gene expression correlates with the programmed appearance of antibody specificities during ontogeny. cDNA libraries will be constructed from mRNA expressed at three stages of development and hybridized with gene-specific probes. (3) To map the chromosomal position of VK subfamilies by field- inversion gel electrophoresis, cosmid cloning and deletion mapping. Rearranged genes from B cells will then be compared to their position on the map to see how rearrangement correlates with chromosomal location.